The need to combat atmospheric pollution has led automobile manufacturers to develop vehicles that consume less fuel and even vehicles without internal combustion engines.
Thus, an increasing number of prototypes and series-manufactured vehicles comprise electric engines, either as their sole driving means or in association with internal combustion engines.
Such vehicles require batteries of electrochemical accumulators to store large quantities of energy in order to power the electric motors. An electrochemical accumulator usually has a nominal voltage of the following magnitude:
1.2 V for NiMH type batteries,
3.3 V for an iron phosphate, lithium-ion, LiFePO4 technology,
4.2 V for a lithium-ion technology based on cobalt oxide.
These nominal voltages are far too low for the requirements of the motor to be powered. To obtain the appropriate voltage level, several electrochemical accumulators are placed in series. To obtain high power and capacity levels, several sets of accumulators are placed in series. The number of stages (number of sets of accumulators) and the number of accumulators in parallel in each stage vary according to the current, the voltage and the capacitance desired for the battery.
The need for a large number of accumulators entails considerable extra cost for the vehicle. Furthermore, such accumulators have a limited service life which generally means that they are replaced at least once during the service life of the vehicle. Since this replacement entails high cost for the user, it is desirable to optimize the service life of the electrochemical accumulators to the maximum.
The conditions of operation and use of the battery have a considerable effect on this service life. Electrochemical accumulators generally operate over a limited temperature range of 0° C. to 60° C. Operation outside this temperature can range cause the accumulators to deteriorate or cause their destruction by thermal runaway. The optimal operating temperature is approximately 30° C. The closer the operation comes to the limits of the operating range, the greater the degradation of the service life of the accumulators. The optimum operating temperature of the accumulators that will guarantee their service life is therefore generally considered to be in the range of 10° C. to 45° C.
Keeping a battery within this range of temperature can prove to be difficult, since automobile vehicles are supposed to work satisfactorily in a temperature range of −30° C. to +45° C. To this end, a certain number of batteries have been developed with circuits for the flow of a liquid to regulate their temperature. Such batteries however prove to be heavier and more complex owing to the presence of the cooling liquid and the obligation of providing tight sealing between this cooling liquid and the electric connector of the accumulators. The ground of the batteries then forms a considerable part of the ground of the vehicle and can impair its performance and its dynamic behavior.
The batteries of motor vehicles also give rise to a certain number of additional design constraints. Automobile batteries must especially be water-tight in order to avoid short-circuits when they are subjected to weather vagaries or even when the vehicle is submerged. In addition, the batteries must be gas-tight. Gas-tightness firstly prevents steam from entering the battery and secondly prevents this steam from condensing when there is a change in altitude or temperature. The condensed water could in this case cause shorting. Furthermore, gas-tightness prevents toxic emanations towards the exterior when there is any accidental deterioration of an accumulator.
Batteries with thermal control by air circulation have also been developed. However, such batteries do not meet the conditions of water-tightness and gas-tightness mentioned here above. The battery especially shows risks of condensation when the air is taken from outside the vehicle or risks to passenger safety when the air is taken from inside the vehicle.
Thus, there is no solution to date that can guarantee the working of the battery in its optimum range of operation with reduced weight and satisfactory conditions of safety.
The document EP2133952 describes a battery provided with electrochemical accumulators. In one particular case, the accumulators are disposed in an adiabatic pack. This pack includes an internal separation between a flow of liquid and a flow of air.
Such a battery does not enable dynamic control, with high operational safety, over the thermal exchanges of the battery with the exterior.